1. Field of the Invention
This invention relates to alloys rich in iron, nickel, cobalt and chromium which form a metastable crystal structure characterized by ultrafine grain size and enhanced compositional uniformity when subjected to rapid solidification processes. Heat treatment of this material causes the precipitation of ultrafine particles (borides, carbides and/or silicides) so as to produce an alloy with desirable mechanical properties.
2. Description of the Prior Art
Rapid solidification processing techniques offer outstanding prospects for the creation of new breeds of cost effective engineering materials with superior properties. (See Proceedings, Int. Conf. on Rapid Solidification Processing, Reston, Virginia, Nov. 1977, published by Claitor's Publishing Division, Baton Rouge, Louisiana, 1978.) Metallic glasses, microcrystalline alloys, highly supersaturated solid solutions and ultrafine grained alloys with highly refined microstructures, in each case often having complete chemical homogeneity, are some of the products that can be made utilizing rapid solidification processing (RSP). [See Rapidly Quenched Metals, 3rd Int. Conf., Vol. 1 & 2, B. Cantor, Ed., The Metal Society, London, 1978. ]
Several techniques are well established in the state of the art to economically fabricate rapidly solidified alloys (at cooling rates of 10.sup.5 -10.sup.7 .degree. C./sec) as ribbons, filaments, wire, flakes or powders in large quantities. Examples include (a) melt spin chill casting, whereby melt is spread as a thin layer on a conductive metallic substrate moving at high speed (see Proc. Int. Conf. on Rapid Solidification Processing, Reston, Virginia, Nov. 1977), and, (b) forced convective cooling by helium gas of centrifugally atomized molten droplets (see Proc. Int. Conf. on Rapid Solidification Processing, Reston, Virginia, Nov. 1977, Baton Rouge, Louisiana).
The current technological interest in materials produced by rapid solidification processing, especially when followed by consolidation into bulk parts, may be traced in part to the problems associated with the chemical segregation that occurs in complex, highly alloyed materials during the conventional processes of ingot casting and processing. During the slow cooling characteristic of casting processes, solute partitioning, i.e. macro and micro-segregation within the different alloy phases present in these alloys, and the formation of undesirable, massive grain boundary eutectics can occur. Metal powders produced directly from the melt by conventional techniques, by inert gas or water atomization of the melt, are usually cooled at rates three to four orders of magnitude lower than those that can be obtained by rapid solidification processing. Rapid solidification processing removes macro-segregation altogether and significantly reduces the spacing over which micro-segregation occurs, if it occurs at all.
Design of alloys made by conventional slow cooling processes is largely influenced by the corresponding equilibrium phase diagrams, which indicate the existence and coexistence of the phases present in thermodynamic equilibrium. Alloys prepared by such processes are in, or at least near, equilibrium. The advent of rapid quenching from the melt has enabled materials scientists to stray further from the state of equilibrium and has greatly widened the range of new alloys with unique structure and properties available for technological applications. Thus, it is known that the metalloid boron has only very low solid solubility in the transition metals Fe, Ni and Co. Alloys of Fe, Ni and Co containing significant amounts of boron, e.g. in the range of 5-10 at%, prepared by conventional technology have at most limited usefulness because they are extremely brittle. This brittleness is due to a network of a hard and brittle eutectic boride phase present along the boundaries of the primary grains of the alloys.
The presence of these hard borides in these alloys could be advantageous if they could be made to be finely dispersed in the matrix metals in the same manner in which certain precipitates are dispersed in precipitation-hardened or dispersion-hardened commerical alloys based on Al, Cu, Fe, Ni, Co and the like.